The invention relates to direct access storage devices such as magnetic disk storage devices. In a magnetic disk storage device, a magnetic disk spins at a high speed while a magnetic recording transducer (a magnetic recording head) is suspended above the disk surface. The head is supported by a slider which floats above the magnetic disk on an "air bearing". The magnetic head writes information onto the magnetic disk in concentric circular tracks. Different tracks are accessed for writing or reading by changing the radial position of the magnetic recording head so as to position the head above the desired track.
One of the most important parameters in a high density magnetic disk storage device is the height (the fly height) of the magnetic recording head above the disk surface while the disk is spinning. Rapid and accurate measurement of this quantity is essential for the development of new slider designs. After the development of new slider designs, such fly height measurement is essential for slider manufacture.
It has been known, in general, to measure the fly height of a magnetic recording head by white light interferometry between a spinning transparent test disk and a slider flying above the test disk. While this technique is sufficiently precise for the current generation of products, the need to measure fly heights to accuracies of a fraction of a wavelength of light threatens to push past the limits of this measurement technique.
In another method of fly height measurement, the slider flies above the surface of an electrically conductive test disk. By measuring the capacitance between the electrically conductive slider and the electrically conductive disk, the fly height can be determined.
The known capacitance measurement technique suffers several drawbacks. Since the entire test disk is conductive, in order to be accurate this technique requires careful accounting for stray capacitance. Moreover, the known measurement technique provides only an average fly height measurement. It provides no detailed measurement of the fly height at different portions of the slider.
For example, the tilt of the slider with respect to the disk cannot be measured by the known capacitance technique. The tilt of the slider is important in order to determine the fly height of the trailing edge of the slider. The fly height of the trailing edge of the slider is important because the trailing edge of the slider is closest to the disk, and because the magnetic recording head is located at the trailing edge of the slider.
Further since the known capacitance measurement technique cannot determine the tilt of the slider, only the relative fly height can be determined. The absolute fly height cannot be determined by measuring the total capacitance between the slider and the conductive disk surface.